In　the　research　of　relay　protection,　transmission　lines　are　usually　imitated　with　different　mathematical　models.　But　the　mathematical　model　only　corresponds　with　actual　physical　model　approximately　when　applied　in　the　special　frequency　band.　So　it　has　to　consider　the　suitable　frequency　band　when　implementing　the　relay　protection.　This　paper　raises　the　definition　of　suitable　frequency　band　of　line　model　applied　to　pilot　differential　protection　based　on　parameter　identification.　And　it　researches　the　suitable　frequency　band　of　transmission　line　Pi　model.　It　appears　that　the　suitable　frequency　band　becomes　narrower　with　the　increase　in　the　length　of　transmission　line,　and　is　irrelevant　with　the　system　impendence.　Finally,　simulations　by　ATP　basically　establish　the　suitable　frequency　bands　of　Pi　model　for　the　transmission　lines　of　different　length,　providing　the　criteria　for　the　design　of　digital　filters　of　pilot　differential　protection　based　on　parameter　identification.